Conventionally, the vehicle interior of a vehicle that runs on an engine (internal combustion engine) is cooled or dehumidified by an air-conditioning unit in which a refrigerant circuit includes a compressor driven by the engine, and heated using engine waste heat.
An electric vehicle that runs on an electric motor and a hybrid vehicle that combines an engine and an electric motor are commercialized in recent years, for the purpose of helping to resolve global environmental issues. The vehicle interior of such a vehicle is cooled or dehumidified by driving an electric compressor included in a refrigerant circuit of an air-conditioning unit with power of a charged battery.
To heat the vehicle interior, on the other hand, engine waste heat can be used in a hybrid vehicle. In a pure electric vehicle, however, engine waste heat is unavailable. The only way is either to draw heat from the outside air by heat pump operation of the air-conditioning unit to heat the vehicle interior, or to use an electric heater. The electric heater has, however, a COP (coefficient of performance) of 1 at the maximum, and consumes a large amount of power stored in the battery. This leads to a shorter cruising range of the electric vehicle.
A COP greater than or equal to 1 can be achieved by heating the vehicle interior by the heat pump operation in which the electric compressor is driven to draw heat from the outside air by an external heat exchanger. In a low outside air temperature environment such as in the middle of winter, however, the temperature difference in pumping is significant and the compression ratio of the compressor is high, and so there is the problem of decreased heating capacity or efficiency.
In view of this, the following air-conditioning unit has been previously developed: a ventilation heat exchanger for recovering heat from the air discharged from the vehicle interior for ventilation is provided to recover heat from the ventilation air in addition to the heat pump from the outside air, thus heating the vehicle interior (see Patent Document 1).